1. Field of the Invention
The present invention relates to a low-coherence reflectometer which uses low-coherence light to measure the reflectance or its distribution in various optical circuits including light guides and optical modules.
2. Description of the Related Art
FIG. 5 is a block diagram showing an outline for the construction of a conventional low-coherence reflectometer. In FIG. 5, numeral 100 represents a low-coherence light source in the form of a light-emitting diode issuing low-coherence light. An end of an optical fiber 101 is connected to the exit end of the low-coherence light source 100. Reference numeral 102 represents a photocoupler having four ports 102a-102d and the other end of optical fiber 101 is connected to the port 102a. A photocoupler 102 receives low-coherence light as an input to the port 102a and splits it at a specified intensity ratio (say, 1:1) into two beams which exit from the ports 102b and 102c. One end of an optical fiber 103 is connected to the port 102b. Connected to the other end of the optical fiber 103 is an optical circuit 104 to be measured.
An optical fiber 105 is connected to the port 102c of the photocoupler 102 and a fiber-type optical isolator 106 is connected to the other end of the optical fiber 105. A fiber-type optical isolator 106 has such characteristics that the input light from the optical fiber 105 is transmitted to an optical fiber 107 connected at the exit end but that the input light from optical fiber 107 is blocked against transmission to the optical fiber 105. The other end of the optical fiber 107 is connected to a port 108a of a photocoupler 108. An optical fiber 109 is connected to a port 108b of the photocoupler 108. Numeral 110 represents a collimator lens preset to have a focal position at the end 109a of the optical fiber 109; numeral 111 represents a reflector mirror for reflecting the light incident via the collimator lens 110 and it is mounted on a stage (not shown) for adjusting the distance to collimator lens 110, An end of an optical fiber 112 is connected to port 108c of the photocoupler 108.
An end of an optical fiber 113 is connected to the port 102d of the photocoupler 102 and the other end of optical fiber 113 is connected to a polarization controller 114. Polarization controller 114 controls the state of polarization of the input light from optical fiber 113. An optical fiber 115 is connected to the exit end of the polarization controller 114. Numeral 116 represents a photocoupler having four ports 116a-116d; connected to the port 116a is the other end of the optical fiber 112 of which an end is connected to the photocoupler 108, and the optical fiber 115 is connected to the port 116b. The photocoupler 116 combines the input light to port the 116a with the input light to the port 116b and issues two beams that exit from the ports 116c and 116d in a specified intensity ratio (say, 1:1). Optical fibers 117 and 118 are connected to the ports 116c and 116d, respectively; the light travelling through optical fiber 117 is subjected to photoelectric conversion by a light-receiving device 119 and the light travelling through the optical fiber 118 is subjected to photoelectric conversion by a light-receiving device 120. Numeral 121 represents a differential amplifier which amplifies the difference between the electrical signals output from the light-receiving devices 119 and 120.
The conventional low-coherence reflectometer having the above-described construction operates as follows. First, the low-coherence light issuing from the low-coherence light source 100 is split by the photocoupler 102 and one branch of the coupler output is picked up as measuring light and launched into the optical circuit 104 via the optical fiber 103, The reflected light produced in the optical circuit 104 is input to the port 102b of the photocoupler 102 via the optical fiber 103 and exits from the port 102d of the photocoupler 102. The reflected light emerging from the photocoupler 102 passes through the polarization controller 114 and is input to the port 116b of the photocoupler 116 via the optical fiber 115.
The other branch of the output light from the photocoupler 102 travels through the optical fiber 105 as local oscillator light and is transmitted through the fiber-type optical isolator 106; thereafter, it is input to the port 108a of the photocoupler 108 via the optical fiber 107. The local oscillator light passes through the photocoupler 108 and optical fiber 109 and exits from its end 109a; the emerging light is converted to parallel light by the collimator lens 110 and incident on the reflector mirror 111. The local oscillator light is then reflected by the reflector mirror 111, converged by the collimator lens 110 and launched into the optical fiber 109 at its end 109a. The local oscillator light entering the optical fiber 109 travels through the photocoupler 108 and optical fiber 112 in that order and is input to the port 116a of the photocoupler 116.
The photocoupler 116 combines the reflected light input to the port 116b with the local oscillator light input to the port 116a. If the optical paths of the measuring light and the reflected light coincide with the optical path of the local oscillator light, interference occurs within the photocoupler 116. The respective branches of the combined light are subjected to photoelectric conversion by the light-receiving devices 119 and 120 and the resulting electrical signals are processed by the differential amplifier 121.
If the stage (not shown) is moved so that the reflector mirror 111 is moved along the optical axis at uniform speed to change the pathlength of the local oscillator light issuing from the photocoupler 108, the amount of group retardation of the local oscillator light is changed. Hence, for each position of the reflector mirror 111, the polarization controller 114 is operated to set the state of polarization of the reflected light to linear polarization at xcex8=0xc2x0 (as being parallel to the paper) and at xcex8=90xc2x0 (as being perpendicular to the paper) and the intensities of the corresponding beat signals I0 and I90 are measured with the differential amplifier 121 and their sum I0+I90 is calculated; in this way, the optical power of the reflected light for each point in the optical circuit 104 can be measured independently of the state of polarization of the reflected light and the local oscillator light, thus making it possible to measure the reflectance distribution. For details of the technology outlined above, see Japanese Patent Laid-Open No. 97856/2000, for example.
In the conventional low-coherence reflectometer described above, the fiber-type optical isolator 106 is provided between the photocouplers 102 and 108 and this is in order to ensure that during measurement of the reflectance distribution in the optical circuit 104, one branch of the local oscillator light emerging from photocoupler the 108 after reflection by the reflector mirror 111 and then travelling through the optical fiber 107 will not reach photocoupler the 102 to be combined there with the reflected light occurring within the optical circuit 104. However, due to the provision of the fiber-type optical isolator 106, the optical path of the local oscillator light starting with the issuance from the port 102c of the photocoupler 102 and ending at the photocoupler 116 where it is combined with the reflected light consists, in the order written, of the optical fiber 105, fiber-type optical isolator 106, optical fiber 107, photocoupler 108, optical fiber 109, collimator lens 110, reflector mirror 111, collimator lens 110, optical fiber 109, photocoupler 108 and optical fiber 112. This is quite a long optical path.
As already mentioned, if the optical paths of the measuring light and the reflected light up to the photocoupler 116 coincide with the optical path of the local oscillator light up to the photocoupler 116, interference occurs within the photocoupler 116. The fiber-type optical isolator 106 provided in the optical path of the local oscillator light causes a corresponding increase in the length of that path. As a result, an optical fiber having a length comparable to the pathlength of the fiber-type optical isolator 106 need be provided in the optical paths of the measuring light and the reflected light. Thus, in the prior art, the prolonged optical fibers are employed and this has caused the problem of increasing the complexity in length adjustment, the layout of components in the apparatus, etc., with the added increase in production cost.
The present invention has been accomplished under these circumstances. An object of the invention is to provide a low-coherence reflectometer in which the overall optical pathlength is shortened to not only facilitate the adjustment of the optical pathlength but also reduce the production cost through simplification of the apparatus and which still is capable of correct measurement of the reflectance distribution and the power of the reflected light.
In order to attain the stated object, the present invention provides a low-coherence reflectometer comprising a light source, a first splitting unit that splits the light from the light source, a second splitting unit by means of which one branch of the output light from the first splitting unit is input as measuring light into an optical circuit to be measured and which splits the reflected light obtained by inputting the measuring light into the optical circuit to be measured, a polarization control unit for controlling the state of polarization of the reflected light as split by the second splitting unit, a third splitting unit by means of which the other branch of the output light from the first splitting unit is allowed to be incident on a reflector mirror as local oscillator light and which splits the local oscillator light reflected by the reflector mirror, and a coupling unit by means of which the reflected light as controlled in the state of polarization by the polarization control unit is combined with the local oscillator light split by the third splitting unit.
The invention also provides a low-coherence reflectometer comprising a light source, a first splitting unit that splits the light from the light source, a second splitting unit by means of which one branch of the output light from the first splitting unit is input as measuring light into an optical circuit to be measured and which splits the reflected light obtained by inputting the measuring light into the optical circuit to be measured, a third splitting unit by means of which the other branch of the output light from the first splitting unit is allowed to be incident on a reflector mirror as local oscillator light and which splits the local oscillator light reflected by the reflector mirror, a polarization control unit for controlling the state of polarization of the reflected light as split by the third splitting unit, and a coupling unit by means of which the reflected light as split by the second splitting unit is combined with the local oscillator light as controlled in the state of polarization by the polarization control unit.
In a preferred embodiment of the invention, the polarization control unit sets the state of polarization in two orthogonal directions.
In another preferred embodiment of the invention, the low-reflectance reflectometer further includes a stage for moving the reflector mirror along the optical axis of the local oscillator light.
In yet another embodiment of the invention, the polarization control unit comprises a Faraday rotator, a coil portion for applying a magnetic field to the Faraday rotator in the direction of light propagation, and a current control portion for controlling the current to the coil portion.
In a further embodiment of the invention, the polarization control unit comprises a half-wave plate and a rotating mechanism for rotating the half-wave plate in a plane orthogonal to the optical axis.
In a still another embodiment of the invention, the polarization control unit comprises a liquid-crystal device capable of setting the pathlength difference between the two principal axes to either zero or half the wavelength and a power supply for adjusting the voltage to the liquid-crystal device.